Random Power chime and dimming - MB Pro
When my MacBook is plugged into power, it randomly will power chime and the screen will darken slightly. It happens very often when plugged into power. So annoying!
MacBook Air 13″, OS X 10.11
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Sounds like there is a problem with the charger, charging cable, or the laptop itself. Try using another USB-C port especially on the other side of the laptop. You can also try rotating the USB-C connector of the charging cable 180 degrees upside down which may work if the USB-C port is half bad. These are just temporary measures to keep using the laptop until it can be repaired since it indicates some sort of hardware issue with the laptop.
You can try an SMC Reset if this is an Intel Mac (review the instructions even if you know how to do this reset since the procedure has changed for later models). You can also try unplugging the charger from the electrical outlet for a minute to reset the charger's internal circuitry. Here is an Apple article for troubleshooting USB-C charging issues:
If your USB-C power adapter isn't charging your Mac laptop - Apple Support
Have Apple or an Apple Authorized Service Provider examine the laptop, charger, and charging cable so they can provide you with a repair estimate.
FYI, it is always best when asking for assistance to provide the exact model of Mac and version of macOS. You can get this information by clicking the Apple menu and selecting "About This Mac".